Contribution Graph
Bars show each included species contribution to log10(Q). Products add positively, reactants subtract.
Current Calculation Snapshot
| Metric | Value |
|---|
Calculator Inputs
Enter gaseous or aqueous activities only. Solids and pure liquids are automatically omitted from Q.
Formula Used
Reaction quotient from concentrations: Qc = ([C]c[D]d) / ([A]a[B]b)
Reaction quotient from partial pressures: Qp = (PCcPDd) / (PAaPBb)
Equilibrium comparison: If Q < K, the reaction tends forward. If Q > K, it tends reverse. If Q ≈ K, it is near equilibrium.
Gibbs relation: ΔG = RT ln(Q/K) when K is supplied. If K is omitted, the calculator reports the RT ln(Q) contribution only.
This page excludes solids and pure liquids because their activities are treated as one in equilibrium expressions. Only gaseous and aqueous species contribute numerically.
How to Use This Calculator
- Choose Qc for concentrations or Qp for partial pressures.
- Enter temperature and, if available, the equilibrium constant K.
- Fill labels, values, coefficients, and phases for products and reactants.
- Mark pure liquids or solids correctly so the calculator omits them.
- Press Calculate Q Value to show results below the header and above the form.
- Download the result summary as CSV or PDF after calculation.
Example Data Table
| Reaction | Basis | Products | Reactants | K | Calculated Q | Direction |
|---|---|---|---|---|---|---|
| N2 + 3H2 ⇌ 2NH3 | Qc | NH3 = 0.50, coeff = 2 | N2 = 0.40, coeff = 1; H2 = 0.20, coeff = 3 | 60 | 78.125 | Reverse toward equilibrium |
| H2 + I2 ⇌ 2HI | Qc | HI = 0.80, coeff = 2 | H2 = 0.50, coeff = 1; I2 = 0.50, coeff = 1 | 2.5 | 2.56 | Near equilibrium |
Frequently Asked Questions
1. What does the chemistry Q value represent?
It is the reaction quotient. Q compares the current mixture composition with the equilibrium expression. It uses the same structure as K, but it is calculated from current values, not equilibrium values.
2. What is the difference between Qc and Qp?
Qc uses molar concentrations, while Qp uses partial pressures. Choose the basis that matches your data and reaction conditions. The mathematical structure stays the same, but the input meaning changes.
3. Why are solids and pure liquids excluded?
Their activities are treated as one in standard equilibrium expressions. Because they do not change the numerical value of Q, this calculator omits them automatically when their phase is selected.
4. How do I know the reaction direction from Q?
Compare Q with K. If Q is smaller, the reaction tends forward to make more products. If Q is larger, it tends reverse. If both are very close, the system is near equilibrium.
5. Can I leave unused species blank?
Yes. Blank species are ignored. The calculator only uses entries that have a value and coefficient together, plus a gaseous or aqueous phase.
6. What units should I use?
Use consistent units within one calculation. Concentrations should be on the same basis for Qc, and partial pressures should be on the same basis for Qp. Mixing inconsistent units can distort the result.
7. Why does the calculator report RT ln(Q) sometimes?
When K is not entered, the calculator cannot compute ΔG from Q/K. It instead reports RT ln(Q), which still shows how the current composition contributes to the Gibbs-energy term.
8. Is this calculator useful for teaching and lab review?
Yes. It helps students and analysts inspect stoichiometric powers, compare Q against K, and visualize which species drive the quotient most strongly in the current mixture.